The present invention relates to a method of forming patterns and, more particularly, to a pattern forming method which is particularly useful to form micropatterns for a semiconductor integrated circuit or a magnetic bubble memory device which has a high integration density.
As is well known, the integration density of a semiconductor integrated circuit or a magnetic bubble memory device in recent years is remarkably increasing, so that the significance of the technology to accurately form various patterns having extremely fine dimensions is increasing more and more.
Various patterns of the semiconductor device and magnetic bubble memory device are formed by the method called photolithography as is well known.
This method is performed in the manner as follows. Namely, a film (photo resist film) whose solubility is varied due to irradiation of the light or X-rays (these radiations are simply written as the light as a general term in this specification) is formed on a film such as an insulation film or conductive film or the like where patterns should be formed. After the light was selectively radiated onto a desired portion of this photo resist film, it is developed to remove the portion having large solubility and the photo resist pattern is formed. The exposed portions of the film where the above-mentioned patterns should be formed are removed by etching, thereby forming various patterns such as wirings and electrodes, etc.
This photolithography is widely used to manufacture various types of semiconductor devices and magnetic bubble memory devices since the micropatterns can be accurately formed. However, when unevenness exist on the substrate under the insulation film and conductive material film, a degree of accuracy upon formation of the patterns remarkably decreases, so that there is a large problem such that it is difficult to accurately form the micropatterns.
The first cause for such an undesirable problem is that the shape of the photo resist pattern becomes inaccurate due to the phenomenon of halation such that the incident light into the photo resist film is reflected by the slant surface at the stepped portions of the substrate and even the portions to which the light should not be inherently radiated are radiated by the above-mentioned reflected light, so that the solubility would have been changed.
Also, the second cause is the phenomenon called the interference effect such that since there is a remarkable difference in thickness in the stepped portions of the photo resist films, a degree of accuracy in formation of the patterns will have been reduced due to the interference by the incident light and the reflected light in the film.
To solve such problems caused due to the uneven portions or different-level portions of the surface of the substrate, a method called "Multi Layer Resist Method" has been generally proposed in the Japanese Patent Application Laid-open No. 107775/76.
According to this method, in the case where the uneven portions or different-level portions exist on the substrate, a thick organic material layer (bottom layer film) is first formed by coating on the film which is processed to make the surface thereof flat, and thereafter a thin film (intermediate film) of SOG (Spin On Glass), PSG (Phosphosilicate Glass), SiO.sub.2, or the like and a thin photo resist film (top layer film) are further formed and laminated thereon.
Subsequently, by the ordinary photolithography technology, the exposure onto the top layer film, development and etching of the intermediate film are performed, thereby processing the intermediate film into a desired shape. Thereafter, the exposed portions of the thick bottom layer film are removed by dry etching using the resulting pattern consisting of the intermediate film as a mask. The exposed portions of the film which is processed are removed by etching, thereby forming a desired pattern.
Although the processing steps of this method are more complicated than the conventional method which uses only the photo resist film of the single layer, the surface of the film is made flat by the presence of the thick bottom layer film and the thin photo resist film is formed thereon, so as to prevent the above-mentioned interference effect in the photo resist film. Further, the light reflected from the different-level portions is absorbed by the bottom layer film and/or intermediate film and doesn't reach the photo resist film (top layer film), so that the influence due to the above-mentioned halation is also prevented.
Thus, even in the case where the uneven portions exist on the surface of the substrate, the wirings and electrodes and other various micropatterns can be formed with high degree of accuracy.
However, in the conventional multi layer resist method, the characteristics of the bottom layer film, intermediate film and top layer film are not sufficiently examined; therefore, the feature of the multi layer resist method is not so perfectly utilized and this results in insufficient accuracy upon formation of the patterns.
With a further increasing integration density of the semiconductor integrated circuit, it is necessary to further highly accurately form further fine patterns and it is strongly desired to further raise a degree of accuracy in the multi layer resist method.